Linear power amplification method and linear power amplifier

ABSTRACT

A combined signal of a digital pilot signal and a digital transmission signal is applied to a digital predistorter ( 20 ), wherein it is added with odd-order distortions based on a power series model to generate a predistorted signal, then the predistorted signal is converted by a DA converter ( 31 ) to an analog signal, then the analog signal is upconverted by a frequency upconverting part ( 33 ) to a send frequency band, and the upconverted signal is output after being amplified by a power amplifier ( 37 ). A pilot signal component is extracted from the power amplifier output, then odd-order distortion components of the power series model are extracted by a digital predistorter control part ( 50 ) from the pilot signal component, and the odd-order distortions in the digital predistorter ( 20 ) are controlled to decrease the levels of the distortion components.

BACKGROUND OF THE INVENTION

The present invention relates to a linear power amplification method anda linear power amplifier for use in a radio communication transmitter,for instance.

One of nonlinear distortion compensating schemes for microwave poweramplifiers is a predistortion scheme using digital signal processing(hereinafter referred to as a digital distortion scheme) (for instance,H. Girard and K. Feher, “A new baseband linearizer for more efficientutilization of earth station amplifiers used for QPSK transmission,”IEEE J. on Selected Areas in Commun. VOL. SAC-1, NO. 1, January 1983). Afeature of the digital predistortion scheme resides in obviating thenecessity of using complex analog circuitry by implementing theoperation of a predistorter through digital signal processing.Conventional linear amplifiers are formed primarily by analog circuitssuch as a feedforward amplifier and a negative feedback amplifier. Thepredistorter is also implemented in analog form (for example, Nojima,Okamoto, and Ohyama, “Predistortion Nonlinear Compensator for MicrowaveSSB-AM System,” Transactions of IEICE of Japan, '84/1 VOL. J67-B NO. 1,pp. 78–85).

Linearization technology using these analog circuits, however, generallycalls for sophisticated adjustment techniques. Furthermore,miniaturization and economization of transmitters including a modulationcircuit require simple configuration of analog circuits. In thisrespect, the digital predistorter, which implements linearizationthrough digital signal processing, is advantageous over the conventionalpredistorter that employs analog circuits. Moreover, an amplifier usingthe predistorter is capable of achieving high efficiency amplificationsince it has no analog circuit for linearization, such as an auxiliaryamplifier used in the feedforward amplifier.

A known configuration of the digital predistorter uses a lookup tablefor pre-linearization of nonlinear characteristics of amplifiers (forexample, L. Sundstrom, IEEE, M. Faulkner, and M. Johansson,“Quantization analysis and design of a digital predistortion linearizerfor RF power amplifiers,” IEEE Trans. Vech. Tech., VOL. 45, NO. 4,pp707–719, November 1996). The digital predistorter using the lookuptable updates set values in the lookup table by feeding back amplifieroutput signals so that distortion components go down below a presetvalue. It is known in the art that distortions can thus be compensatedby digital signal processing and that the compensated amount ofdistortion is approximately 15 dB or below (Y. Oishi, N. Tozawa, and H.Suzuki, “Highly Efficient Power Amplifier for IMT-2000 BTS Equipment,”FUJITSU Sci. Tech. J., 38, 2, p. 201–208, December 2002). To maximizethe efficiency of amplification by the power amplifier, it is necessaryto compress the output backoff of the amplifier by increasing the amountof distortion to be compensated for. FIG. 1 shows the relationshipbetween the output backoff from a 1-dB gain compression point and theefficiency of amplification. The condition for review is an ideal class“B” bias. From FIG. 1, it will be seen that greater amplificationefficiency can be achieved by increasing the amount of distortion to becompensated for to such an extent as to enable compression of the outputbackoff.

FIG. 2 shows the relationships between the distortion reduction andamplitude and phase deviations of a third-order distortion component. Toachieve distortion compensation performance at least above 30 dB, adigital predistorter is needed which yields an amplitude deviationwithin ±0.2 dB and a phase deviation within ±2 deg. As will be seen fromFIG. 2, the digital predistorter is required to attain predeterminedamplitude and phase deviations in accordance with secular andtemperature variations as well.

To realize distortion compensation (distortion improvement) in excess ofa value attainable at present, the conventional lookup table typedigital predistorter needs to be equipped, as will be understood fromFIG. 3, with a high-precision lookup table for maintaining thedistortion compensation at a high level. Further, it is necessary toprovide a control route which, when a nonlinear characteristic of thepower amplifier slightly changes with a temperature deviation or secularvariation, monitors the amplifier output signal and corrects the lookuptable accordingly.

On the digital predistorter using the lookup table, however, therelationships between distortion components and values set in the lookuptable have not been clarified nor has been presented any concrete methodfor correcting a slight variation in the nonlinear characteristic of theamplifier that is caused by a secular or temperature change, forinstance.

One approach to high-precision compensation for distortion components isa predistorter based on a power series model. Such a predistorter hasbeen implemented so far using analog circuits, and its distortionimprovement performance is above 30 dB (for instance, T. Nojima and T.Konno, “Cuber predistortion linearizer for relay equipment in 800 MHzband land mobile telephone system,” IEEE Trans. Vech. Tech., VOL. VT-34,NO.4, pp169–177, November 1985). It is known in the art that the powerseries model is one that models nonlinear characteristics of theamplifier with high precision (for example, Tri T. Ha, “Solid-StateMicrowave Amplifier Design,” Chapter 6, Krieger Publishing Company,1991). With the distortion compensation scheme of the digitalpredistorter using the power series model, signals for correctingcoefficients of respective orders need to be extracted from theamplifier output signal. In British Patent Application PublicationGB2335812A there is described the extraction of such correction signalsby removing distortion component of the fundamental wave and higherorders from the transmission signal. A scheme for more easy extractionof the correction signals of the power series model is to use twocarriers of the same levels as pilot signals. (see the afore-mentioneddocument by T. Nojima and T. Konno).

There have been proposed improving the frequency dependence of thenonlinear characteristic of the power amplifier as well as compensationfor its temperature dependence. With a view to implementing excellentcompensation for distortion in a wideband signal by the conventionalpredistorter, Japanese Patent Application Publication No. 11-17462proposes reduction of the path difference between the main signal pathand the distorted signal path, and Japanese Patent ApplicationPublication No. 7-7333 proposes the connection of a phase equalizer tothe input signal line. The reason for using such schemes is to cause thedistortion generated by the predistorter to vary with a fixed gain andin a fixed phase over a wide frequency band.

However, widening of the frequency band for amplification providesincreased frequency deviation in the gain and phase characteristics ofthe power amplifier as shown in FIG. 3, for instance,—this exertsnonnegligible influence on signal amplification. On this account, onlyby fixedly varying the amplitude and phase of the distortion over theentire frequency band, it is impossible that the distortion by thepredistorter remains over the entire frequency band at a level forcanceling the distortion by the power amplifier and opposite thereto inphase. Accordingly, to implement high-precision distortion compensation,it is necessary that frequency dependent amplitude and phasecharacteristics of the distortion by the predistorter be varied in sucha manner as to cancel frequency deviations of gain and phasecharacteristics of the power amplifier. Japanese Patent ApplicationPublication No. 10-327209 proposes the use of an equalizer to vary thefrequency-amplitude and frequency-phase characteristics of thedistortion generated by the predistorter.

For example, in the conventional predistorter shown in Japanese PatentApplication Publication No. 2002-64340, the output from an analogdistorter is adjusted in amplitude and phase at the higher- andlower-frequency sides of the fundamental wave output signalindependently of each other to impart frequency characteristics to thedistortion for compensation. In Japanese Patent Application PublicationNo. 2002-57533 an amplitude-frequency characteristic adjusting circuitcomposed of a band-pass filter and a vector adjuster is connected to theoutput side of an analog distorter so that the distortion forcompensation has a frequency characteristic.

In the case of extracting an intermodulation distortion component of theamplifier output by a narrow-band filter and correcting each ordercoefficient of the analog predistorter, the coefficient can easily becorrected in a sufficiently short time for the transmission signal in apilot signal feedback route in the analog predistorter. In contrast tothe analog predistorter, the lookup table type digital predistorterinvolves digitization of the pilot signal monitored from the amplifieroutput, giving rise to a problem of delay in the feedback route.

In the analog predistorter the pilot signal is generated by an analogoscillator, whereas in the digital predistorter the pilot signal needsto be generated in the base band through digital signal processing. Noconcrete techniques or schemes have been proposed so far for signalconversion of the pilot signal and the transmission signal in thedigital predistorter and for their analog-to-digital conversion.

In other words, it is still unclear how to configure the digitalpredistorter that uses the pilot signal. There is a demand for a simpleconfiguration of the digital predistorter that achieves a high degree ofdistortion compensation and always performs distortion compensationaccording to secular and temperature variations.

The scheme of varying the frequency characteristic of the distortiongenerated by the predistorter through use of an equalizer, described inthe afore-mentioned Japanese Patent Application Publication No.10-327209, is to make uniform the frequency characteristics of thefeedback route that controls the predistorter. This scheme does not takeinto consideration the frequency deviations of the gain and phasecharacteristics in the power amplifier. Accordingly, there arises thenecessity for a predistorter capable of adjusting thefrequency-amplitude and frequency-phase characteristics of thedistortion generated by the predistorter in such a manner as to cancelthe frequency deviations of the gain and phase characteristics in hepower amplifier.

When the input signal is one that has discrete spectra on the frequencyaxis as in the case of using two carriers of the same amplitude, it iseffective to impart the frequency characteristic to the distortioncomponent by adjusting its amplitude and phase on the higher- andlower-frequency sides of the fundamental wave signal as proposed in theafore-mentioned Japanese Patent Application Publication No. 2002-64340.With this method, however, when the input signal has a continuousspectrum on the frequency axis like a modulated wave signal, it isimpossible to provide the distortion component with such frequencycharacteristics that it continuously varies on the frequency axis. Inthe afore-mentioned Japanese Patent Application Publication No.2002-57533 many band-pass filters and vector adjusters need to beprepared for imparting frequency characteristics to high-orderdistortions for compensation, too. Besides, it is also still unclear howto implement the frequency characteristics of compensating distortionsfor canceling the frequency characteristics of distortion componentgenerated by the power amplifier. The predistorters disclosed in theafore-mentioned Patent Application Publication Nos. 2002-64340 and2002-57533 are predistorters formed by analog elements. In thisinstance, implementation of the frequency characteristics for thecompensating distortions calls for taking into account the frequencycharacteristics of the entire transmission system including thedistorter, the vector adjuster and so on, as well as the frequencycharacteristics of the power amplifier.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide a linear poweramplification method and a linear power amplifier which are not muchaffected by secular and temperature changes and achieve excellentdistortion compensation performance.

The linear power amplifier according to the present invention comprises:

a digital predistorter supplied with a digital transmission signal, forpredistorting said digital transmission signal by use of a power seriesmodel to generate a predistorted signal;

a digital-to-analog converter for converting said predistorted signalfrom said digital predistorter into an analog predistorted signal;

a frequency upconverting part for upconverting said analog predistortedsignal to a transmit frequency band;

a power amplifier for power-amplifying said upconverted signal;

a frequency downconverting part for downconverting a portion of theoutput from said power amplifier to output a downconverted signal; and

a digital predistorter control part for extracting distortion componentsof the same odd orders as those of said power series model and forcontrolling coefficients of said predistorter in a manner to lower thelevels of said odd-order distortion components.

Since the odd-order distortion components of the power series model tobe generated in the digital predistorter are directly controlled toreduce the levels of the extracted distortion components, a distortioncorrection with small secular and temperature variations can beachieved.

The linear power amplification method according to the present inventioncomprises the steps of:

(a) inputting a digital pilot signal to a digital predistorter togenerate a predistorted signal added with odd-order distortioncomponents of a number predetermined by a power series model;

(b) converting said predistorted signal to an analog predistortedsignal;

(c) upconverting said analog predistorted signal to a transmit frequencyband by use of a predetermined carrier frequency;

(d) power-amplifying said upconverted signal;

(e) downconverting a portion of said power-amplified output signal toextract odd-order distortion components; and

(f) controlling coefficients of said predistorter so that the levelratio of said odd-order distortion component to a transmission signalbecomes smaller than a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between amplificationefficiency and an output backoff from a 1 dB gain compression point;

FIG. 2 is a graph showing the relationships between amplitude and phasedeviations in respect of a third-order distortion component;

FIG. 3 is a graph showing examples of frequency dependent amplitude andphase of a power amplifier;

FIG. 4 is a block diagram illustrating a basic configuration of thelinear power amplifier according to the present invention;

FIG. 5 is a block diagram depicting a first embodiment of the linearpower amplifier according to the present invention;

FIG. 6 is a diagram schematically showing spectra of signals atrespective parts in FIG. 5:

FIG. 7 is a flowchart showing the procedure for implementing the linearpower amplification method according to the present invention;

FIG. 8 is a block diagram depicting a second embodiment of the linearpower amplifier according to the present invention;

FIG. 9 is a diagram schematically showing spectra of signals atrespective parts in FIG. 8;

FIG. 10 is a block diagram depicting a third embodiment of the linearpower amplifier according to the present invention;

FIG. 11 is a block diagram depicting a fourth embodiment of the linearpower amplifier according to the present invention;

FIG. 12 is a block diagram depicting a modified form of each of thethird and fourth embodiments of the linear power amplifier according tothe present invention;

FIG. 13 is a block diagram showing a modified form of the FIG. 10embodiment;

FIG. 14 is a block diagram showing a modified form of the FIG. 12embodiment;

FIG. 15 is a block diagram illustrating another example of a digitalpredistorter control part;

FIG. 16A is a diagram depicting an equivalent circuit of a FET;

FIG. 16B is a diagram depicting an equivalent circuit of an amplifierusing a FET;

FIG. 17 is a block diagram illustrating a basic configuration of a fifthembodiment of the linear power amplifier according to the presentinvention;

FIG. 18 is a graph for explaining the operation of the fifth embodiment;

FIG. 19 is a block diagram showing a concrete example of the fifthembodiment;

FIG. 20 is a block diagram depicting the configuration of a sixthembodiment of the present invention;

FIG. 21 is a block diagram depicting the configuration of a seventhembodiment of the present invention;

FIG. 22 is a block diagram showing a modified form of the FIG. 21embodiment in which a pilot signal generator 12 generates a modulationsignal as a pilot signal;

FIG. 23 is a block diagram depicting the configuration of an eighthembodiment of the present invention;

FIG. 24 is a flowchart showing the procedure for calculatingcharacteristics of a frequency characteristic compensator;

FIG. 25 is a frequency chart for explaining the generation ofcompensated distortions for a third-order distortion;

FIG. 26A is a graph showing a grain-frequency characteristic of afrequency characteristic compensator calculated by linear interpolation;

FIG. 26B is a graph showing a phase-frequency characteristic of thefrequency characteristic compensator;

FIG. 27A is a graph showing a gain-frequency characteristic of thefrequency characteristic compensator calculated by polynomialinterpolation;

FIG. 27B is a graph showing a determined phase-frequency characteristicof the frequency characteristic compensator;

FIG. 28A is a graph showing a combined gain-frequency characteristic ofa frequency characteristic compensator and a gain adjuster, calculatedby linear interpolation;

FIG. 28B is a graph showing a combined phase-frequency characteristic ofthe frequency characteristic compensator and a phase adjuster;

FIG. 29A is a graph showing a combined gain-frequency characteristic ofthe frequency characteristic compensator and the gain adjuster,calculated by polynomial interpolation;

FIG. 29B is a graph showing a combined phase-frequency characteristic ofthe frequency characteristic compensator and the phase adjuster;

FIG. 30 is a block diagram illustrating the configuration of a ninthembodiment of the present invention;

FIG. 31 is a block diagram illustrating the configuration of a tenthembodiment of the present invention;

FIG. 32 is a block diagram illustrating the configuration of an eleventhembodiment of the present invention;

FIG. 33 is a flowchart showing the procedure for setting characteristicsof the frequency characteristic compensator in the FIG. 32 embodiment;and

FIG. 34 is a block diagram showing a modified form of the FIG. 32embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Basic Constitution of the Invention

FIG. 4 illustrates a basic constitution of the linear power amplifieraccording to the present invention. A transmission signal S and a pilotsignal PL are generated through different digital signal processing by atransmission signal generator 11 and a pilot signal generator 12,respectively, and they are added together by an adder 15, the adderoutput being provided to a digital predistorter 20. The transmissionsignal S may be either a baseband or IF signal; but it will hereinafterbe assumed as a baseband signal unless otherwise specified. The digitalpredistorter 20, based on a power series model, performs digital signalprocessing for predistorting the input signal that is a combined signalof the transmission signal S and the pilot signal PL.

The output signal from the digital predistorter 20 is converted to ananalog signal by a digital-to-analog (DA) converter 31 that has aworking speed in a band at least twice higher than the band of thecombined signal of the pilot signal PL and the transmission signal S.The analog signal is frequency converted by a frequency upconvertingpart 33 to a high-frequency signal of the transmit frequency band, andthe frequency-converted signal is fed to a power amplifier 37. Theoutput signal from the power amplifier 37 is divided by a power dividingpart 38 into two, one of which is provided to a frequency downconvertingpart 40 and the other of which is provided as a linear amplifier outputto, for example, an antenna. The one divided portion of power isdownconverted in the frequency downconverting part 40, thereafter beingfed to a digital predistorter control part 50. The control part 50extracts an odd-order distortion component of the pilot signal from thedownconverted signal, and uses the extracted distortion component tocorrect coefficients of the digital predistorter 20.

Since the digital predistorter 20 using the pilot signal does notcorrect the coefficients by use of correction data read out of a memorybut instead directly corrects the coefficients by use of the detecteddistortion component in such a manner as to reduce the distortioncomponent as referred to above, the coefficient correction is free fromthe influence of secular and temperature variations. Further, as regardsa pilot signal feedback time, since the band of the pilot signal isnarrower than the band of the transmission signal, the delay time of thedigital predistorter in the present invention can be extended longerthan the delay time in a conventional digital predistorter. Accordingly,the feedback time does not matter even in the feedback route in whichthe pilot signal is downconverted as shown in FIG. 4.

First Embodiment

FIG. 5 illustrates a first embodiment of a linear power amplifierembodying the digital predistortion scheme according to the presentinvention. The pilot signal used in this embodiment is two tone signalsPL₁ and PL₂ of the same level. The linear power amplifier of thisembodiment comprises: a pilot signal generator 12 composed of tonesignal generators 12A and 12B for generating the tone signals PL₁ andPL₂ through digital signal processing, and a digital adder 14; a digitalpredistorter 20; a DA converter 31; a frequency upconverting part 33composed of a local oscillator 33A, a mixer 33B, and a band-pass filter33C; a power amplifier 37; a directional coupler 38A and a pilot signalextracting band-pass filter 38B that constitute a dividing part 38; afrequency downconverting part 40 composed of a mixer 41, a band-passfilter 42, an amplifier 43, and an analog-to-digital (AD) converter 44;and a digital predistorter control part 50. The digital predistorter 20is shown to handle distortions of up to the seventh order, but thenumber of orders may be chosen as desired according to the deviceconfiguration used. While in practice a low-pass filter for aliasingcutting use is connected to the output side of the DA converter 31, itis not related directly to the present invention and hence is not shown.

The digital predistorter 20 using a power series model is configured toadd output signals from a delay path which passes therethrough thefundamental wave component of the transmission signal and the path forgenerating each odd-order distortion by use of the power series. Thatis, the fundamental wave component passes through a delaying memory 21which provides coincidence between the delay times of the delay path andthe distortion generation path. Distortion components of the respectiveodd orders are produced by distortion generators 22A, 22B and 22C, gainadjusters 24A, 24B and 24C for amplitude adjustment use, and phaseadjusters 23A, 23B and 23C for phase adjustment use. The odd-orderdistortion generators 22A, 22B and 22C each perform processing ofraising the input combined signal of the transmission signal A and thepilot signals PL₁ and PL₂ to the corresponding odd-order power. Forinstance, letting X represent the sum of the transmission signal S andthe pilot signals PL₁ and PL₂, the third-order distortion generatorraises X to 3rd power. The phase- and amplitude-adjusted odd-orderdistortion components are added together by adders 26 and 27, then theadded output is further added by an adder 25 to the delayed fundamentalwave component from the delaying memory 21, and the added output isapplied as a predistorted signal Y from the digital predistorter 20 tothe DA converter 31.

The DA converter 31 converts the predistorted signal Y to an analogsignal, which is applied to the mixer 33B, wherein it is mixed with alocal signal (a carrier signal) of a frequency f_(c) fed from the localoscillator 33A. The mixed output is provided to the band-pass filter 33Cto extract a signal of the transmit frequency band, which is applied tothe power amplifier 37. The output high-frequency signal from the poweramplifier 37 is transmitted via the directional coupler 38A.

A portion of the transmit output of the high-frequency signal is takenout by the directional coupler 38A and is applied to the band-passfilter 38B to extract a pilot signal component (composed of pilotsignals and higher order distortions). The thus extracted pilot signalcomponent is mixed by the mixer 41 with the carrier signal from thelocal oscillator 33A, and the mixer output is applied to the band-passfilter 42 to detect a downconverted pilot signal component, which isamplified by the amplifier 43. The amplified pilot signal component isconverted by the DA converter 44 to a digital signal, which is providedto the digital predistorter control part 50.

The digital predistorter control part 50 comprises a distortioncomponent detecting part 51 and an odd-order distortion characteristiccontrol part 52. The distortion component detecting part 51 is made upof third-, fifth- and seventh-order distortion component extractors 51A,51B and 51C. The odd-order distortion characteristic control part 52 ismade up of third-, fifth- and seventh-order distortion controllers 52A,52B and 52C. The odd-order distortion component extractors 51A, 51B and51C can be formed, for example, by band-pass filters, by which third-,fifth- and seventh-order distortion components are extracted. Theodd-order distortion controllers 52A, 52B and 52C control the phaseadjusters 23A, 23B, 23C and the gain adjusters 24A, 24B, 24C that adjustthe phases and amplitudes of the outputs from the distortion componentgenerators 22A, 22B and 22C corresponding to the controllers,respectively.

Since the pilot signals PL₁ and PL₂ used are tone signals of the samelevel (CW signals), odd-order distortion components appearing in thevicinities of the tone signals are extracted at the output of the poweramplifier 37 by the odd-order distortion component extractors 51A, 51Band 51C. While the digital predistorter control part 50 in thisembodiment is implemented by digital signal processing, a similarconfiguration may be implemented by analog circuits.

FIG. 6 shows, in the form of signal spectra, how to inject and extractthe pilot signals PL₁ and PL₂ in this embodiment. The input signal X tothe digital predistorter 20 contains the transmission signal S of thebaseband and the pilot signals PL₁ and PL₂ that are tone signals of thesame level. The pilot signals PL₁ and PL₂ of frequencies f₁ an f₂ areinjected into the adjacent band of the transmission signal S as shown inFIG. 6-Row A. The two pilot signals PL₁ and PL₂ are set with a frequencyinterval Δf=f₂−f₁ which is sufficiently narrower than the modulatedsignal bandwidth of the transmission signal S. The output signal Y fromthe digital predistorter 20 contains predistorted components S_(D),P_(D3L) and P_(D3H) resulting from predistortion of the transmissionsignal S and the pilot signals PL₁ and PL₂, as shown in FIG. 6-Row B.Here are exemplified the third-order distortion components; for example,the fifth-order distortion components of the pilot signals PL₁ and PL₂are a component higher than P_(D3H) by Δf and a component lower thanP_(D3L) by Δf, but they are not shown. The seventh-order distortioncomponents are generated further outside than the fifth-order distortioncomponents by Δf, but they are not shown, either.

The input signal to the power amplifier 37 is a signal that the outputsignal Y from the digital predistorter 20 was upconverted in thefrequency upconverting part 33 by the carrier frequency f_(c) asdepicted in FIG. 6-Row C. In this case, the predistorted componentsgenerated by the digital predistorter 20 are so set as to compensate fordistortions over the entire transmission route. Accordingly, no problemarises from a mismatch between the predistorted components in the inputsignal to the power amplifier 37 and the predistorted components in theoutput signal of the digital predistorter 20. But the difference is verysmall since intennodulation distortions in the transmit route mostlyoccur in the power amplifier 37 at the final stage of the route. Asshown in FIG. 6-Row D, the output signal from the power amplifier 37 isa signal with distortions suppressed by the digital predistorter 20,that is, a distortion-compensated signal.

The pilot signal component containing the distortion components isextracted by the directional coupler 38A and the band-pass filter 38B.The extracted pilot signal component is downconverted by the mixer 41with the local oscillation signal from the local oscillator 33. Theinput signal to the control part 50, shown in FIG. 6-Row E, is adigitized version of the downconverted signal by the AD converter 44,For example, when distortion compensation for the third-order distortioncomponents P_(D3H) and P_(D3L) is insufficient at the output of thepower amplifier 37, they remain unremoved to such an extent as not to benegligible. In the control part 50 one of the third-order distortioncomponents, P_(D3H) in this case, is extracted by the third-orderdistortion component extractor 51A. The third distortion controller 52Auses the extracted tone signal to control the phase and amplitude of theoutput from the third distortion signal generator 22A by the phaseadjuster 23A and the gain adjuster 24A until the compensated amount ofdistortion reaches such a value that the adjacent channel leakage powerratio (i.e., the level ratio of the distortion component to thetransmission signal) goes down below a predetermined value at the outputof the power amplifier 37. To perform this, various optimal algorithmscan be used.

FIG. 7 is a flowchart showing a linear power amplification procedureincluding the steps for setting coefficients in the digital predistorter20 by controlling the phases of the phase adjusters 23A, 23B, 23C andthe gains of the gain adjusters 24A, 24B, 24C.

Step S1: Generate digital pilot signals PL₁ and PL₂, and add them with adigital transmission signal S to obtain a combined signal.

Step S2: Generate odd-order distortion components for the digitalcombined signal.

Step S3: Set the phases and amplitudes of the odd-order distortioncomponents.

Step S4: Add the distortion components and the delayed fundamental wavecomponent to generate a predistorted signal.

Step S5: Convert the predistorted signal to an analog signal.

Step S6: Upconvert the analog predistorted signal to a high-frequencysignal.

Step S7: Power amplify the high-frequency predistorted signal by a poweramplifier.

Step S8: Extract the pilot signal components from the amplifiedhigh-frequency signal and downconvert them.

Step S9: Convert the downconverted pilot signal components to digitalform.

Step S10: Extract distortion components from the digital pilot signalcomponents.

Step S11: Make a check to see if the ratio of the distortion componentlevel to the transmission signal level is below a predetermined value,and if so, end the procedure, and if not, return to step S3 and repeatsteps S3 through S11.

Second Embodiment

FIG. 8 illustrates in block form a second embodiment of the presentinvention, which is a modified form of the first embodiment. Theillustrated embodiment employs one modulated wave signal as the pilotsignal instead of using the two tone signals, and is identical inconstruction with the first embodiment except the configuration of thepilot signal generator 12. And this embodiment is also identical inoperation with the first embodiment.

FIG. 9 shows, in the form of signal spectra, the injection andextraction of the pilot signal PL in the second embodiment. Rows A and Bschematically show spectra of the input signal X to and the outputsignal Y from the digital predistorter 20, Rows C and D spectra of theinput signal to and the output signal from the power amplifier 37, andRow E spectrum of the input signal to the control part 50. The spectrashown in FIG. 9 are identical with those in FIG. 6 except that the pilotsignal PL in the second embodiment is a modulated signal. The pilotsignal PL is a modulated signal having a bandwidth, which is distortedby the digital predistorter 20 and has its spectrum spread on both sidesaccordingly as indicated by P_(D). As compared with the pilot signalsPL₁ and PL₂ which are tone signals, the detection sensitivity of thepilot signal PL in this embodiment is increased by a decoding circuitwhich performs error correction or the like in the receiver. Theapplication of a spreading code to the pilot signal permits extractionof a pilot signal below the lowest receiving sensitivity of thereceiver.

Third Embodiment

FIG. 10 illustrates in block form a third embodiment of the presentinvention, which differs from the first and second embodiments in thatpredistorters 20 ₁, 20 ₂ and DA converters 31 ₁ and 31 ₂ providedseparately for the pilot signal and the transmission signal. The digitalpredistorters 20 ₁, 20 ₂ and the digital predistorter control part 50therefor are identical in construction with those in the first andsecond embodiments.

In this embodiment there are newly provided a frequency upconvertingpart 34 composed of a local oscillator 34A, a mixer 34B and a band-passfilter 34C, for frequency converting the output from the second digitalpredistorter 20 ₂ to a band different from that of the transmissionsignal S. This embodiment contemplates widening the band of thetransmission signal. The first and second embodiments permits reductionof computational complexities for predistortion, generation andinjection of the pilot signals and digital signal processing, butwidening the band of the transmission signal is likely to cause shortageof the capacity of the DA converter 31. Further, since the pilot signalis injected into a band different from that of the transmission signalS, the DA converter 31 is required to be capable of performingdigital-to-analog conversion of signals in bands above that of thetransmission signal. In this respect, the third embodiment usesdifferent digital predistorters 20 ₁, 20 ₂ and different DA converters31 ₁, 31 ₂ for the transmission signal and the pilot signal,respectively. The provision of such independent digital-to-analogconversion routes offers increased flexibility in widening of thetransmission signal or signal conversion for over sampling. The firstand second digital predistorters 20 ₁ and 20 ₂ synchronously correctcoefficients of each odd order under the control of the digitalpredistorter control part 50.

Fourth Embodiment

FIG. 11 illustrates in block form a fourth embodiment of the presentinvention, in which the pilot signal generator 12 in the fourthembodiment of FIG. 10 has the same configuration as that of the pilotsignal generator 12 for generating a modulated signal in the FIG. 8embodiment. This embodiment is also identical in operation with thethird embodiment. As compared with the pilot signals PL₁ and PL₂ whichare tone signals, the detection sensitivity of the pilot signal PL inthis embodiment is increased by a decoding circuit which performs errorcorrection or the like in the receiver. The application of a spreadingcode to the pilot signal permits extraction of a pilot signal below thelowest receiving sensitivity of the receiver.

In the third and fourth embodiments of FIGS. 10 and 11, the first andsecond digital predistorters 20 ₁ and 20 ₂ may also be replaced with onedigital predistorter. In such an instance, a band separator 30 isprovided, as shown in FIG. 12, which performs signal processing forseparating the transmission signal and the pilot signal at the output ofthe digital predistorter 20 through utilization of the difference inband between the transmission signal and the pilot signal. Thetransmission signal S and the pilot signal PL thus separated areprocessed in the respective routes in the same manner as in FIGS. 10 and11.

In the embodiments of FIGS. 10, 11 and 12 the transmission signal andthe pilot signal are predistorted and converted from digital to analogform separately of each other, and the predistorted pilot signal isupconverted and combined with the predistorted transmission signal. FIG.13 shows a modified form of the FIG. 10 embodiment. In this embodimentthe predistorted transmission signal upconverted in the frequencyupconverting part 33 by use of the carrier frequency f_(c) and thepredistorted pilot signal upconverted in the frequency upconverting part34 by use of a carrier frequency f_(c)′ different from theabove-mentioned carrier frequency f_(c) are combined by an adder 35, andthe combined signal is applied to the amplifier 37. Further, the carriersignal of the carrier frequency f_(c)′ from th local oscillator 34A isapplied to the mixer 41 of the pilot signal component detecting part 40to detect the pilot signal component. The illustrated modification isidentical in construction and in operation with the FIG. 10 embodimentexcept the above.

It is apparent that the embodiments of FIGS. 11 and 12 can also bemodified as in the case of FIG. 13. For example, in the FIG. 11embodiment the pilot signal generator 12 the pilot signal generator 12for generating two tone signals in FIG. 13 is replaced with a pilotsignal generator for generating a modified signal of a band narrowerthan that of the transmission signal. In the case of the FIG. 12embodiment the circuit arrangement following the band separator 30 needsonly to be the same as shown in FIG. 14; no description will be repeatedin this respect.

FIG. 15 shows, by way of example, a circuit configuration for increasingthe pilot signal detecting sensitivity of the digital predistortercontrol part 50 in the first to fourth embodiments and in theirmodifications. In this case, however, the pilot signal generator 12 isone that combines two tone signals into the pilot signal as referred topreviously in respect of FIG. 5. The FIG. 15 example is directed only tothe third-order distortion.

The digital predistorter control part 50 comprises a third-orderdistortion component extractor 50A and a third-order distortioncontroller 52A. The third-order distortion component extractor 50Acomprises: a delay memory 1A11, a phase adjuster 1A12 and a gainadjuster 1A13 which constitute a fundamental wave generating path; afifth-order distortion generator 1A21, a phase adjuster 1A22 and a gainadjuster 1A23 which constitute a fifth-order distortion generating path;a seventh-order distortion generator 1A31, a phase adjuster 1A32 and again adjuster 1A33 which constitute a seventh-order distortiongenerating path; and subtractors 1A14, 1A24 and 1A34.

From the pilot signal component fed from the pilot signal generator 12are generated a delayed fundamental wave component, a fifth-orderdistortion component and a seventh-order distortion component throughthe fundamental wave path, the fifth-order distortion generating pathand the seventh-order distortion generating path, respectively. Thedelayed fundamental wave component, fifth-order distortion component andseventh-order distortion component of the pilot signal are sequentiallysubtracted by the subtractors 1A14, 1A24 and 1A34, respectively, fromthe pilot signal component detected in the frequency downconverting part40, by which the third-order distortion component is left remaining, andthe third-order distortion component is provided to the third-orderdistortion controller 52A. Based on the third-order distortion componentfed thereto, the third-order distortion controller 52Acontrols the phaseadjuster 23A and the gain adjuster 24A of the digital predistorter 20 asis the case with the third-order distortion controller 52A in FIG. 5.

To reduce residues of the delayed fundamental wave component, the fifthdistortion component and the seventh-order distortion component afterthe subtraction, the control part 50 of FIG. 15 adjusts the phases andamplitudes of the respective components by the phase adjusters 1A12,1A22, 1A32 and the gain adjusters 1A13, 1A23, 1A33. These adjustmentsneed only to be made at the time of device initialization becauseimplementation of the digital predistorter control part 50 of FIG. 15 bydigital signal processing does not cause any changes in electricalcharacteristics due to aging or temperature. With the same configurationas that of the digital predistorter control part 50 in FIG. 8, it ispossible to extract the fifth- or seventh-order distortion component.The same goes for the case where the pilot signal is a modulated signal.

Fifth Embodiment

An equivalent circuit of an intrinsic region of a common FET (FieldEffect Transistor) used in the power amplifier can be expressed, forexample, as shown in FIG. 16A, in which C_(gs) represents thegate-source interterminal capacitance, R_(g) the gate resistance, G_(m)the transconductance, and G_(d) the drain conductance. Theintermodulation distortion in FET is modeled in the form of power seriesof C_(gs), G_(m) and G_(d) from the FIG. 16A equivalent circuit of theintrinsic region (see, for example, J. A. Higgins and R. L. Kuvas,“Analysis and improvement of intermodulation distortion in GaAs powerFET's,” IEEE Transaction on Microwave Theory and Techniques, VOL.MTT-28, NO. 1, pp. 9–17, January 1980). Letting an instantaneous gatevoltage be represented by V_(g) and an instantaneous drain voltage byV_(d),C _(m)(v _(g))=G _(m1) +G _(m2) V _(g) +G _(m3) V _(g) ² +G _(m4) V _(g)³ +G _(m5) V _(g) ⁴+  (1)G _(d)(V _(d))=G _(d1) +G _(d2) V _(d) +G _(d3) V _(d) ² +G _(d4) V _(d)³ +G _(d5) V _(d) ⁴+  (2)C _(gs)(V _(g))=C _(g1) +C _(g2) V _(g) +C _(g3) V _(g) ² +C _(g4) V_(g) ³ +C _(g5) V _(g) ⁴+  (3)From the above it is understood that the intermodulation distortion inFET occurs at the gate and the drain.

The amplifier can be expressed in the form of such a circuit network asshown in FIG. 16B by use of FET equivalent circuit of FIG. 16A. Thecircuit network is composed of a gate-side matching circuit 37A, FET anda drain-side matching circuit 37A. The matching circuits 37A and 37Bhave different frequency characteristics. Because of this, theintermodulation distortion of the amplifier is affected by the frequencycharacteristics of the both gate- and drain-side matching circuits 37Aand 37B. But the frequency characteristics in this case are not so widein bandwidth as the operating frequency of FET, and they are limited tothe bandwidth for amplification by the amplifier.

In the conventional power series type predistorter by digital signalprocessing, no consideration is given to the frequency characteristicsof the intermodulation distortion in FET (see, for instance, UK PatentApplication GB2335812A).

With a view to achieving a high degree of distortion suppression over awide band, this embodiment compensates for the intermodulationdistortion taking into account the frequency characteristics of thegate-side matching circuit 37A and the frequency characteristics of thedrain-side matching circuit 37B separately of each other. What isimportant in FIG. 16B is that the input signal to the amplifier isinfluenced by the frequency characteristics of the gate-side matchingcircuit 37A and then applied to the FET equivalent circuit, wherein theintermodulation distortion is generated. That is, the input signal towhich causes the intermodulation distortion is attributable comes underthe influence of the frequency characteristics of the gate-side matchingcircuit 37A. Similarly, the frequency characteristics of the drain-sidematching circuit 37B affects the distortion generated by FET.

Accordingly, to compensate for the frequency characteristics of thedistortion generated by FET, a frequency characteristic compensator isprovided at the input side of each odd-order distortion generator in thepower series predistorter, by which it is possible to compensate for thefrequency characteristics of the distortion in conformity to thegate-side frequency characteristics of the amplifier. That is, theprovision of the frequency characteristic compensator at the input sideof each odd-order distortion generator implements the frequencycharacteristics that compensate for the frequency characteristics of thegate-side matching circuit 37A at the output of the power amplifier.

Similarly, by placing a frequency characteristic compensator at theoutput side of each odd-order distortion generator in the digitalpredistorter, it is possible to provide compensation for the frequencycharacteristics of the distortion that conforms to the drain-sidefrequency characteristics of the amplifier. That is, the provision ofthe frequency characteristic compensator at the output side of eachodd-order distortion generator implements the frequency characteristicsthat compensate for the frequency characteristics of the drain-sidematching circuit 37B at the output of the power amplifier. That is, theprovision of the frequency characteristic compensator at the output sideof each odd-order distortion generator implements the frequencycharacteristics that compensate for the frequency characteristics of thedrain-side matching circuit 37B at the output of the power amplifier.

For example, the frequency characteristics T(f) of the intermodulationdistortion by the gate-side matching circuit are expressed by thefollowing equation (4) using Eq. (3).T(f)C _(g)(V _(g))=T ₁(f)C _(g1) +T ₂(f)C _(g2) V _(g) +T ₃(f)C _(g3) V_(g) ² +T ₄(f)C _(g4) V _(g) ³ +T ₅(f)C _(g5) V _(g) ⁴  (4)From Eq. 84) it will be seen that the digital signal processing typepredistorter needs to compensate for the frequency characteristics foreach odd-order distortion generator. The same goes for the drain side.Further, the intermodulation distortions occur simultaneously at thegate-side and drain-side of FET, and the power series type predistorterconfiguration differ with the magnitude of each of the intermodulationdistortions defined by Eqs. (1) to (3). The frequency characteristics ofthe intermodulation distortion by the amplifier described above inrespect of FIG. 3 can be considered to be a combined version of thegate-side and drain-side frequency characteristics. The frequencycharacteristic compensator is provided so that frequency characteristicsinverse to the combined frequency characteristics is imparted to theoutput from each odd-order distortion generator. The frequencycharacteristic compensator is placed at that one of the input and outputterminal sides of FET at which the intermodulation distortion isdominant, or at either side. When it is disposed only at the output orinput side of the odd-order distortion generator, the frequencycharacteristic compensator for compensating for the combined frequencycharacteristics of the intermodulation distortion cannot always achievesatisfactory compensation; in some cases, however, the compensationperformance can be improved by placing the compensator at either of theinput and output sides of the odd-order distortion generator.

In this embodiment, the frequency dependence of the distortionsuppression by the digital signal processing type predistorter isimproved by using a frequency characteristic compensator whichcompensates for only the frequency characteristics of the gate-sidematching circuit 37A and/or frequency characteristic compensator whichcompensates for the frequency characteristics of both the gate- anddrain-side matching circuits 37A and 37B.

FIG. 17 illustrates in block form a basic configuration of the fifthembodiment according to the present invention. The basic configurationincludes: a digital predistorter 20 for predistorting the transmissionsignal S from the transmission signal generator 11; a DA converter 31for converting the predistorted output to an analog transmission signal;a frequency upconverting part 33 for upconverting the analogtransmission signal to a high-frequency transmission signal; a poweramplifier 37 for amplifying the upconverted transmission signal; and adividing part 38 for dividing the amplified output into two; a frequencydownconverting part 40 for downconverting one of the two dividedoutputs; a distortion component detecting part 51 for detecting anodd-order distortion component from the downconverted signal; and acontroller 5 for controlling a phase adjuster 23 and a gain adjuster 24based on the detected odd-order distortion component. The distortioncomponent detecting part 51 and the controller 5 constitute the digitalpredistorter control part 50.

Moreover, in the digital predistorter 20 there is placed at the outputside of a distortion generator 22 a frequency characteristic compensator28 by which a characteristic inverse to the frequency characteristic ofthe power amplifier 37, shown in FIG. 3, is imparted to the distortiongenerated by the distortion generator 22, and the frequencycharacteristics for the distortion is controlled by he controller 5.

The input signal S from the transmission signal generator 11 is branchedto a linear transfer path 2L and a distortion generating path 2D of thepredistorter 20. The power-series-model distortion generator 22generates an odd-order distortion signal D by use of the input signalbranched to the distortion generating path 2D. The frequencycharacteristic compensator 28 adjusts frequency dependent amplitude andphase characteristics of the distortion signal D to be inverse to thefrequency characteristics of the amplifier 37. The output from thefrequency characteristic compensator 28 is adjusted in phase and in gainby the phase adjuster 23 and the gain adjuster 24 to yield an adjusteddistortion signal D′, which applied to the combiner 25. The signalbranched to the linear transfer path 2L is provided to the delay memory21, wherein the amount of delay is corrected relative to the signal onthe distortion generating path 2D. The combiner 25 combines the signalsS and D′ from the linear transfer path 2L and the distortion generatingpath 2D.

In this configuration, too, in order to maintain a high degree ofdistortion compensation for a characteristic change of the poweramplifier 37 by a temperature change or aging, the output from the poweramplifier 37 is monitored by he distortion compensation detecting part51 via the dividing part 38, and upon detecting a reduction in thedistortion compression effect by the distortion component detecting part51, the controller 5 changes parameters of the phase adjuster 23, thegain adjuster 24 and the frequency characteristic compensator 28. Thisensures constant maintenance of a high degree of distortioncompensation. Incidentally, as will be understood from the descriptiongiven of FIGS. 16A and 16B, the frequency characteristic compensator 28may be provided at the input side of the distortion generator 22 or ateither of the input and output sides thereof as indicated by the brokenline in FIG. 17.

Turning now to FIG. 18, a description will be given of the principle onwhich high-precision distortion compensation can be implemented by thefrequency characteristic compensator 28 in FIG. 17. Assume that whensupplied with the input signal S shown on Row B, the power amplifier 37of the frequency characteristics shown on Row A generates such adistortion D_(S) as shown on Row C. To cancel such a distortion D_(S),the frequency characteristics of the frequency characteristiccompensator 28 are rendered inverse to the frequency characteristics ofthe power amplifier 37 as shown on Row B, by which the frequencydependent amplitude and phase characteristics of the distortion D by thedistortion generator 22 shown on Row D are adjusted to obtain adistortion D′ shown on Row F. The gain adjuster 24 adjusts the gain ofthe distortion produced by the distortion generator so that it has alevel at which the distortion D_(S) generated by the power amplifier 37can be canceled, and the phase adjuster 23 adjusts the frequencycharacteristics of the frequency characteristic compensator 28 to beinverse to the frequency characteristics of the power amplifier 37. D′indicates the adjusted characteristics. As a result, the output S+D′from the combiner 25 is a combined version of thefrequency-characteristic-compensated distortion D′ and the signal S asshown on Row G. The combined signal S+D′ applied via the DA converter 31to the power amplifier 37, by which the frequency characteristics of thepower amplifier 37 can be cancelled; accordingly, in the output S_(A)from the power amplifier 37 there is cancelled the distortion as shownon Row H.

FIG. 19 illustrates in block form a concrete embodiment based on thebasic configuration depicted in FIG. 17. This embodiment comprises: adigital predistorter 20; a DA converter 31; a frequency upconvertingpart 33 composed of a local oscillator 33A, a mixer 33B and a band-passfilter 33C; a power amplifier 37; a directional coupler 38A and a signalextracting band-pass filter 38B forming a signal extracting part 38; amixer 41 and a band-pass filter 42 forming a frequency downconvertingpart 40; and a digital distorter control part 50. The frequencydownconverting part 40 includes an AD converter 44 for converting adownconverted extracted signal to a digital signal. The digitalpredistorter 20 is shown to handle the third, fifth and seventh-orderdistortion components, but the number of orders may be chosen as desiredaccording to the device configuration used.

The digital predistorter 20 using a power series model is configured toadd output signals from a delay path which passes therethrough thefundamental wave component of the transmission signal and the path forgenerating each odd-order distortion by use of the power series. Theodd-order distortion generators 22A, 22B and 22C each perform processingof raising the input transmission signal to the corresponding odd-orderpower. For instance, letting x represent the transmission signal, thethird-order distortion generator raises x to 3rd power. The frequencycharacteristic compensators 28A, 28B and 28C are FIR (Finite ImpulseResponse) filters, and their coefficients are set and controlled by thecoefficient controllers 53A, 53B and 53C. The output distortion signalsfrom the distortion generators 22A, 22B and 22C are input to the FIRfilters 28A, 28B and 28C, by which frequency dependent amplitude andphase characteristics of the distortion signals can be varied.

The output signal from the power amplifier 37 is extracted by thedirectional coupler 38A and the band-pass filter 38B, and the extractedsignal is downconverted by the frequency downconverting part 40. Theinput signal to the digital predistorter control part 50 is a digitizedversion of the downconverted signal by the AD converter 44. The digitalpredistorter control part 50 is made up of odd-order distortioncomponent extractors 51A, 51B and 51C each formed by a distortioncomponent extracting band-pass filter; distortion controllers 52A, 52Band 52C corresponding to the respective odd-order distortion components;and coefficient controllers 53A, 53B and 53C for controlling thecoefficients of the FIR filters 28A, 28B and 28C of the respective oddorders. The odd-order distortion controllers 52A, 52B and 52C eachcontrol the corresponding ones of the gain adjusters 24A, 24B, 24C andthe phase adjusters 23A, 23B, 23C for the outputs from the distortiongenerators 22A, 22B and 22C in the digital predistorter 20.Incidentally, the odd-order distortion controllers 52A, 52B, 52C and thecoefficient controllers 53A, 53B, 53C constitute the distortioncharacteristic controller 5 in FIG. 17.

The FIR coefficient controllers 53A, 53B and 53C for the respective oddorders each control the coefficients of the corresponding one of the FIRfilters 28A, 28B and 28C. The odd-order distortion component extractors51A, 51B and 51C each extract the corresponding one of the odd-orderdistortion component signal by a band-pass filter or the like. Theodd-order distortion controllers 52A, 52B and 52C use the extractedsignals to control, based on the outputs from the odd-order distortiongenerators 22A, 22B and 22C, the gain adjusters 52A, 52B, 52C and thephase adjusters 23A, 23B, 23C until the compensated amount of distortionreaches such a value that the adjacent channel leakage power ratio(i.e., the level ratio of the distortion component to the transmissionsignal) goes down below a predetermined value at the output of the poweramplifier 37. At the same time, the frequency characteristics of thepower amplifier 37 are extracted, and the coefficients of the respectiveFIR filters 28A, 28B and 28C accordingly. The parameter control can beimplemented by use of various optimal algorithms. In the embodiment ofFIG. 19, too, the FIR filters 28A, 28B and 28C may connected to only tothe inputs or both the inputs and outputs of the third- fifth- andseventh-order distortion generators 22A, 22B and 22C as indicated by thebroken lines.

Sixth Embodiment

FIG. 20 illustrates in block form an embodiment of a linear poweramplifier that uses FFT's as the frequency characteristic compensators28A, 28B and 28C of the digital predistorter 20. This embodiment is amodified form of the FIG. 19 embodiment, which uses, as each of thefrequency characteristic compensators 28A, 28B and 28C (represented by28A in this case), a set of an FFT (Fast Fourier Transform) part 28A1, acoefficient multiplier 28A2 and an IFFT (Inverse Fast Fourier Transform)part 28A3, instead of using the FIR filter. The same goes for thefrequency characteristic compensators 28B and 28C. Except the above thisembodiment is identical in construction with the FIG. 19 embodiment.Accordingly, the frequency characteristic control part 53 has thecoefficient controllers 53A, 53B and 53C corresponding to the third-,fifth- and seventh-order distortions as is the case wit the frequencycharacteristic control part 53 in the FIG. 19 embodiment, but they arenot shown. The same goes for the embodiments of FIGS. 21, 30 and 31described later on.

For example, the distortion signal from the third-order distortiongenerator 22A is applied to the FFT part 28A1, wherein it is Fouriertransformed for each of plural samples to frequency domain samples. Theamplitude of the sample at each frequency point is multiplied, by thecoefficient multiplier 28A2, by the coefficient from the coefficientcontroller 53A, and the multiplied output is inverse Fast Fouriertransformed by the IFFT part 28A3 into a time domain sample. The samegoes for the other frequency characteristic compensators 28B and 28C.The frequency characteristic control by FFT is implemented bycontrolling each multiplication coefficient of FFT as mentioned above.The digital predistorter control part 50 controls the gain adjuster, thephase adjuster and the multiplication coefficient of FFT for each oddorder so that the level of the distortion component by the poweramplifier 37 relative to the transmission signal goes down below apredetermined value. In this embodiment, too, the frequencycharacteristic compensators 28A, 28B and 28C may be connected only tothe inputs or both of the inputs and outputs of the third-, fifth- andseventh-order distortion generators 22A, 22B and 22C as indicated by thebroken lines.

Seventh Embodiment

FIG. 21 illustrates in block form a seventh embodiment of the presentinvention. This embodiment is a modified form of the FIG. 19, which isconfigured to make an adjustment to the digital predistorter 20 by useof the two pilot signals shown in FIG. 5. The odd-order distortiongenerators 22A, 22B and 22C of the digital predistorter 20 using a powerseries model each performs processing of raising the transmission signaland the pilot signal input thereto the corresponding odd order.

The digital predistorter control part 50 is identical in constructionwith the digital predistorter control part 50 in each of the embodimentsof FIGS. 19 and 20. The odd-order distortion controllers 52A, 52B and52C control the gain adjusters 24A, 24B, 24C and the phase adjusters23A, 23B, 23C of those of the distortion component generators 22A, 22Band 22C of the digital predistorter 20 which correspond to thecontrollers 52A, 52B and 52C, respectively. The coefficient controllers53A, 53B and 53C (not shown) of the frequency characteristic controlpart 53 control the coefficients of the frequency characteristiccompensators 28A, 28B and 28C, respectively. Since two tone signals ofthe same level as the pilot signal, odd-order distortion componentsappearing near the tone signals at the output of the power amplifier 37are extracted by the odd-order distortion component extracting band-passfilters functioning as the distortion component extractors 51A, 51B and51C of the respective odd orders. While the digital predistorter controlpart 50 in this embodiment is implemented by digital signal processing,it may also be formed by analog circuits. The frequency characteristiccompensation utilizes the distortion components P_(D3L) and P_(D3H) thatappear in the bands lower and upper than that of the pilot signal asdescribed previously in respect of FIG. 6.

The frequency characteristic compensators 28A, 28B and 28C may be formedby the FIR filters as in the FIG. 19 embodiment; alternatively, they mayeach be formed using the FFT parts, the coefficient multiplier and theIFFT part. The frequency characteristic compensation corrects thefrequency characteristics by use of the upper and lower distortionsignals P_(D3H) and P_(D3L) in FIG. 6. For example, the coefficientcontrollers 53A, 53B and 53C each interpolate the detected values of theupper and lower distortion components P_(D3H) and P_(D3L) from thecorresponding one of the odd-order distortion component extractors 51A,51B and 51C to thereby estimate the frequency characteristics from amonitor value. The FIR filters or FFT's forming the frequencycharacteristic compensators 28A, 28B and 28C each set the interpolatedvalue in the corresponding coefficient multiplier. Thereafter eachfilter or FFT adjusts the multiplication coefficient until apredetermined distortion suppression-frequency characteristic isobtained. The required control can be effected by various optimalalgorithms.

Even if modulated waves are used as the pilot signals in place of thetone signals, the same results as mentioned above are obtainable.Furthermore, a different predistorter may also be used for each of thepilot signal and the transmission signal. In this embodiment, too, thefrequency characteristic compensators 28A, 28B and 28C may be connectedonly to the inputs or both of the inputs and outputs of the third-,fifth- and seventh-order distortion generators 22A, 22B and 22C asindicated by the broken lines.

FIG. 22 illustrates in block form a modified form of the FIG. 21embodiment, in which the pilot signal generator 12 generates a modulatedsignal as in the FIG. 8 embodiment. Since this embodiment is identicalin construction with the FIG. 21 embodiment except the above, nodescription will be repeated. Besides, the digital predistorter in theFIG. 12 embodiment and the digital predistorters 20 ₁ and 20 ₂ in theembodiments of FIGS. 13 and 14 may also be modified to have the sameconfiguration as that of the digital predistorter 20 shown in FIG. 21,for instance, and the digital predistorter control part 50 in each ofthe embodiments shown in FIGS. 12, 13 and 14 may also be configuredsimilar to the control part 50 in FIG. 21.

Eighth Embodiment

FIG. 23 illustrates in block form a basic configuration of amodification of the FIG. 17 embodiment adapted to adjust the digitalpredistorter 20 by use of two pilot signals shown in FIG. 5. In thisembodiment there are additionally provided a pilot signal generator 12for generating two pilot signals PL₁ and PL₂ and an adder 15 for addingtogether the pilot signals and the transmission signal S, and thecontroller 5 controls the pilot signal generator 12 to vary thefrequency interval between the pilot signals PL₁ and PL₂ of the sameamplitude. In the digital predistorter control part 50 there is provideda storage part 55 for storing the gain and phase obtained from thefrequency characteristics of the detected distortion component.

As referred to previously, the transmission signal S may be a basebandsignal or IF signal. In the latter case, it is recommended that thefrequencies of the pilot signals PL₁ and PL₂ be set at f_(IF)−f_(i)/2and f_(IF)+f_(i)/2, respectively, with respect to a predeterminedintermediate frequency f_(IF). When the transmission signal S is abaseband signal, a signal A cos πf_(i)t of an amplitude A and afrequency f_(i)/2 is subjected to quadrature modulation in the frequencyupconverting part 33 by use of a carrier signal of a frequency f_(c);that is, by obtaining the real part of the result of multiplication ofcos πf_(i)t by (cos 2πf_(c)t+jsin 2πf_(c)t), the two pilot signals offrequencies f_(IF)−f_(i)/2 and f_(IF+f) _(i)/2 are generated in thetransmission frequency band. Accordingly, the pilot signal generator 12needs only to generate a tone signal of a frequency f_(i)/2 in practice.Since the signal expressed by cos πf_(i)t can be regarded as havingpositive and negative frequency components, as given by the followingequationcos πf _(i) t=(exp jπf _(i) t+exp−jπf _(i) t)/2  (5)the frequencies of the two pilot signal PL₁ and PL₂ in the baseband willhereinafter be expressed by −f_(i)/2 and +f_(i)/2, respectively.

The intermodulation distortions, which are created when the pilotsignals upconverted in the frequency upconverting part 33 are amplifiedby the power amplifier 37, are detected in the distortion componentdetecting part 51 of the digital predistorter control part 50 via thedividing part 38 and the frequency downconverting part 40. Thecontroller 5 adjusts the parameters of the gain adjuster 24, the phaseadjuster 23 and the frequency characteristic compensator 28 so that theintermodulation distortions go down below a predetermined value of theadjacent channel leakage power ratio. The use of two pilot signalsfacilitates extraction of the odd-order distortion components modeled bythe power series, allowing ease in adjustment of the frequencycharacteristic compensator 28, the gain adjuster 24 and the phaseadjuster 23 in the digital predistorter 20.

Through variations of the two pilot signal frequencies −f_(i)/2 and+f_(i)/2 by the controller 5, the frequency interval f_(i) between thetwo upconverted pilot signals in the transmit frequency band undergoescorresponding variations, causing changes in the frequency of occurrenceof each intermodulation distortion on the frequency axis accordingly.Thus, by changing the pilot signal frequencies −f_(i)/2 and +f_(i)/2 atfixed intervals, it is possible to determine the gain and phase of eachcompensation distortion that achieves a predetermined adjacent channelleakage power ratio for the frequency of occurrence of each resultingintermodulation distortion.

By interpolating gains and phases discretely obtained on the frequencyaxis by the above method, continuous frequency characteristics for thecompensation distortions can be obtained. The thus obtained frequencycharacteristics are implemented by the frequency characteristiccompensator 28 and imparted to the compensation distortions.

FIG. 24 is a flowchart showing the procedure for obtaining thecharacteristics of the frequency characteristic compensator 28, whichwill be described below with reference to the frequency diagram of FIG.25.

Step S1: Initialize the value of a variable i at 1.

Step S2: Generate two digital tone signals of baseband frequencies−f_(i)/2 and +f_(i)/2 (hence, spaced f_(i) apart) and equal in amplitudeas the pilot signals PL₁ and PL₂ (FIG. 25-Row A). These signal arecombined, then the combined signal is upconverted with the centerfrequency f_(c) in the frequency upconverting part 33, and when theupconverted signal is input to the power amplifier 37, intermodulationdistortions P_(D3H) and P_(D3L) of frequencies f_(c)+3f_(i)/2 andf_(c)−3f_(i)/2, for example, expressed by the following equations occurat the output of the power amplifier 37 (Row B):B _(3H) cos 2π(f _(c) +f _(i)/2+f _(i))t=B _(3H) cos 2π(f _(c)+3f_(i)/2)t  (6)B _(3L) cos 2π(f _(c) −f _(i)/2−f _(i))t=B _(3L) cos 2π(f _(c)−3f_(i)/2)t  (7)where B_(3H) and B_(3L) represent the amplitudes of distortions atfrequencies upper and lower than the carrier frequency f_(c),respectively.

To cancel the intermodulation distortions P_(D3H) and P_(D3L), thepredistorter 20 outputs a signal with compensation distortions D_(L)′and _(D)H′ added to the pilot signals PL₁ and PL₂ (Row C). This signalis upconverted in the frequency upconverting part 33, and theupconverted signal is applied to the power amplifier 37. The outputsignal from the power amplifier 37 becomes a signal compensated for bythe digital predistorter 20 (Row D). The gain adjuster 24, the phaseadjuster 23 and the frequency characteristic compensator 28 are adjustedin a manner to cancel the intermodulation distortions P_(D3H) andP_(D3L). Incidentally, the gain adjuster 24 impart a fixed gain G tofrequency, and the phase adjuster 23 impart a fixed phase change P tofrequency.

Step S3: Set the gain G in the gain adjuster 24 and the phase P in thephase adjuster 23. These values may be set as desired, but maypreferably be set such that the adjacent channel leakage power ratiobecomes relatively small.

Step S4: Extract the third-order intermodulation distortions in theoutput from the power amplifier 37 by the distortion component detectingpart 51, and make a check to see if the upper and lower intermodulationdistortions P_(D3H) and P_(D3L) each meet the requirement that theadjacent channel leakage power ratio be smaller than a predeterminedvalue. If only the upper or both the upper and lower distortions do notmeet the requirement, go to step S5. When only the lower distortion doesnot satisfy the requirement, go to step S7, and when either distortionsatisfies the requirement, go to step S9.

Step S5: If the upper or both of the upper and lower distortions P_(D3H)and P_(D3L) do not the above-mentioned requirement, the gain G_(i) andphase P_(i) corresponding to the frequency f_(c)+3f_(i)/2 of thefrequency characteristic compensator 28 are each varied aspredetermined.

Step S6: Make a check to see if the upper distortion P_(D3H) meets therequirement, and if not, return to step S5 and repeat the sameprocessing. When the upper distortion satisfies the requirement, go backto step S4 and make the check again.

Step S7: When only the lower distortion P_(D3L) does not satisfy therequirement, the gain G_(i)′ and phase P_(i)′ corresponding to thefrequency f_(c)−3f_(i)/2 of the frequency characteristic compensator 28are each varied as predetermined.

Here, assume that the gains G_(i) and G_(i)′ of the frequencycharacteristic compensator 28 represent differences from the gain G ofthe gain adjuster 24 and that the phases P_(i) and P_(i)′ aredifferences from the phase change P of the phase adjuster 23.

Step S8: Make a check to see if the lower distortion P_(D3L) meets therequirement, and if not, go back to and repeat step S7. If thedistortion meets the requirement, make sure in step S4 that the upperand lower distortions both satisfy the requirement, and go to step S9.Alternatively, skip step S4 and proceed directly to step S9.

Step S9: When the upper and lower distortions P_(D3H) and P_(D3L) bothsatisfy the requirement that the adjacent channel leakage power ratio besmaller than a predetermined value, store the gains G₁, G₁′ and thephases P₁, P₁′ at that time in a storage part 55, and determine if i=N.

Step S10: If not i=N, then increment i by 1 and return to step S2. Andset the frequency intervals between the pilot signals set at f₂ (in theexamples of FIGS. 26A, 26B, 27A and 27B described later on, thefrequency spacing f_(i) decreases with an increase in the variable i),and as is the case with the frequency spacing f₁, perform steps S3through S9 to obtain the gains and phases G₂, G₂′ and P₂, P₂′ of thefrequency characteristic compensator 28 each of which satisfies therequirement that the adjacent channel leakage power ratio be smallerthan a predetermined value, and store them in the storage part 55. Inthis instance, the values of the gain adjuster 24 and the phase adjuster23 are fixed at G and P, respectively.

By repeating N rounds of processing while changing the frequencyinterval between the two pilot signals PL₁ and PL₂ from i=1 to i=N, G₁to G_(N), G₁′ to G_(N)′, P₁ to P_(N) and P₁′ to P_(N)′ are stored in thestorage part 55.

Step S11: Obtain the frequency characteristics for compensationdistortion by use of the values G₁ to G_(N), G₁′ to G_(N)′, P₁ to P_(N)and P₁′ to P_(N)′ obtained as described above. The frequencycharacteristics can be obtained by interpolating the gains G₁ to G_(N),G₁′ to G_(N)′ and the phases P₁ to P_(N) and P₁′ to P_(N)′ between pointas shown FIG. 26A, 26B, or 27A, 27B. FIGS. 26A and 26B show linearinterpolation, and FIGS. 27A and 27B show polynomial interpolation; butother interpolation schemes, such as spline and Lagrangean interpolationschemes can also be used.

By interpolating the discretely obtained gains and phases as mentionedabove, the frequency characteristics of the distortion component areimplemented by the frequency characteristic compensator 28. The ultimatefrequency characteristics for the distortion component are a combinedversion of the frequency characteristics of the gain adjuster 24, thephase adjuster 23 and the frequency characteristic compensator 28. Forhigher-precision distortion compensation, the frequency spacing of thepilot signals is further reduced. While the above description has beengiven of the third-order distortion alone, the above-described methodcan be used for compensation for fifth- or higher-order distortion aswell.

Ninth Embodiment

FIG. 30 illustrates a more specific configuration of the FIG. 23embodiment, which uses the digital predistorter 20 in which thefrequency characteristic compensators 28A, 28B and 28C are formed by FIRfilters as in the FIG. 19 embodiment. Let is be assumed that the pilotsignal generator 12 generates digital tone signals PL₁ and PL₂ ofvariable frequencies expressed by −f_(i)/2 and +f_(i)/2. The digitalpredistorter control part 50 further includes a frequency controller 54for controlling the oscillation frequency f_(i) of the pilot signalgenerator 12 in correspondence to the control by the frequencycharacteristic control part 53. This embodiment is identical inconstruction with the FIG. 19 embodiment except the above.

The two tone signals spaced f_(i) apart and equal in amplitude are inputas the pilot signals PL₁ and PL₂ to the digital predistorter 20, whichoutputs a signal with compensation distortions added to the pilotsignals. The output signal is converted by the DA converter 31 to ananalog signal, which is applied to the frequency upconverting part 33and upconverted therein to a high-frequency carrier signal of the centerfrequency f_(c). The high-frequency signal is amplified by the poweramplifier 37. The compensation distortions created by the digitalpredistorter 20 are so set as to provide distortion compensationthroughout the transmission route. Accordingly, the compensationdistortions in the input signal to the power amplifier 37 and in theoutput signals from the digital predistorter may differ from each other.That is, a desired device for changing the phase and amplitude of thesignal may be inserted between the output of the digital predistorter 20and the input of the power amplifier 37.

As is the case with the FIG. 19 embodiment, the intermodulationdistortion component is extracted by the directional coupler 38A and theband-pass filter 38B, and downconverted in the frequency downconvertingpart 40. The input signal to the digital predistorter control part 50 isa digitized signal of the downconverted signal. The compensation for thethird-order distortion will be described below by way of example. Thethird-order distortion component extractor 51A extracts, by upperband-pass filter and a lower band-pass filter, the upper and lowerintermodulation distortion signals that are third-order distortioncomponents. The gain adjuster 24A, the phase adjuster 23A and thefrequency characteristic compensator 28A use the extracted signals tovary the amplitude and phase of the output from the third-orderdistortion signal generator until the distortion compensation reachessuch a value that the adjacent channel leakage power ratio at the outputof the power amplifier 38 goes down below a predetermined value.

The procedure of obtaining these compensation parameters begins withsetting the gain G of the gain adjuster 24A and the phase P of the phaseadjuster 23A as referred to previously in respect of FIG. 23. Thesevalues may be set as desired, but may preferably be set such that theadjacent channel leakage power ratio becomes relatively small.

Next, the gain G₁ and the phase P₁ of the frequency characteristiccompensator 28A at the upper frequency (f_(c)+3f₁/2) and the gain G₁′and the phase P₁′ at the lower frequency (f_(c)−3f₁/2) are adjusted sothat the adjacent channel leakage power ratio becomes lower than apredetermined value. This can be done by use of various optimizationalgorithms such as the least square estimation method and the steepestdescent method. Next, the frequency interval between the two pilotsignals of the same amplitude is changed to f₂, and G₂, G₂′ and P₂, P₂′are calculated. This procedure is repeated N times to obtain those gainsand phases G₁ to G_(N), G₁′ to G_(N)′, P₁ to P_(N) and P₁′ to P_(N)′ ofthe frequency characteristic compensator 28 for the frequencies f₁ tof_(N) which satisfy the requirement that the adjacent channel leakagepower ratio be smaller than a predetermined value. The thus obtainedgain and phase values can be interpolated using the linear, polynomial,Lagrangean, or spline interpolation scheme. The tap coefficients of theFIR filter are se by the controller such that the gain and frequencycharacteristics obtained by interpolation are implemented.

While the above description has been given only of the third-orderdistortion, compensation for fifth- higher-order distortion can also beachieved by the above-described method. In such a case, intermodulationdistortion corresponding to the odd-order distortion to be compensatedfor is extracted. The FIR filters 28A, 28B and 28C may also be disposedat the input sides of the odd-order distortion generators 22A, 22B and22C.

The amplitude and phase of the distortion component in the output fromthe power amplifier 37 varies due to temperature or aging. Therefore, toprovide high-precision compensation for distortions at all times, it isnecessary to adaptively control setting of the gain adjusters 24A, 24B,24C, the phase adjusters 23A, 23B, 23C and the frequency characteristiccompensators 28A, 28B, 28C. In this embodiment the use of two pilotsignals enables their adaptive control.

Tenth Embodiment

FIG. 31 illustrates in block form a modified form of the FIG. 30, inwhich the frequency characteristic compensators 28A, 28B and 28C areformed using the three sets of FFT part, coefficient multiplier and IFFTpart 28A1, 28A2, 28A3-28B1, 28B2, 28B3-28C1, 28C2, 28C3 as in the FIG.20 embodiment, instead of using the FIR filters. As described previouslyin respect of FIG. 20, the output signals from the distortion generators22A, 22B and 22C are converted to frequency domain signals by Fouriertransform processing in the FFT parts 28A1, 28B1 and 28C1, then thefrequency domain signals are multiplied by the frequency compensationcharacteristics by the coefficient multipliers 28A2, 28B2 and 28C2, andthe multiplied output signals are inversely transformed to time domainsignals in the IFFT parts 28A3, 28B3 and 28C3. The digital predistortercontrol part 50 controls gain adjusters 2A, 24B, 24C, and the phaseadjusters 23A, 23B, 23C, and the multiplication coefficients of thefrequency characteristic compensators 28A, 28B and 28C so that thedistortion components in the output from the power amplifier 37 eachachieve the predetermined adjacent channel leakage power ratio. Themethod for setting the coefficients of the frequency characteristiccompensators 28A, 28B and 28C by use of the pilot signal is the same asdescribed previously in respect of the FIG. 30 embodiment.

The configuration of the digital predistorter 20 and the digitalpredistorter control part 50 shown in FIGS. 10 and 20 (or FIGS. 30 and31) may also be applied to he two digital predistorters 20 ₁, 20 ₂ andthe configuration of the digital predistorter control part 50 in theembodiments of FIGS. 10, 22 and 13. Similarly, the configuration of thedigital predistorter 20 and the configuration of the digitalpredistorter control part 50 shown in FIGS. 19 and 20 (or FIGS. 30 and31) may also be applied to the digital predistorter 20 and the digitalpredistorter control part 50 in FIGS. 12 and 14.

While in the above FIR filters have been described to be used as thefrequency characteristic compensators, they may be replaced with IIR(Infinite Impulse Response ) filters.

Eleventh Embodiment

The embodiment FIG. 23 and the embodiments of FIGS. 30 and 31 areconfigured to determine the frequency characteristics of the frequencycharacteristic compensators 28 (29A, 28A, 28C) while sequentiallychanging the frequency intervals Δf between the two pilot signals PL₁and PL₂ of the same amplitude. In the embodiment described below,however, the frequencies of the pilot signals are fixed, and thefrequencies of the high-frequency pilot signals are sequentially changedby steps of Δf in the working band of the power amplifier bysequentially changing the upconverting frequency, then intermodulationdistortions at the respective frequencies are detected, and thecharacteristics of the frequency characteristic compensator 28 aredetermined accordingly.

This embodiment will be described below with reference to FIG. 32.

As is the case with the FIG. 30 embodiment, this embodiment comprises: apilot signal generator 12; an adder 15; a digital predistolter 20; a DAconverter 31; a frequency upconverting part 33; a power amplifier 37; adivider 38; a frequency downconverting part 40; and a digitalpredistorter control part 50.

This embodiment differs from the FIG. 30 embodiment in that instead ofchanging the frequencies of the pilot signals, a frequency controller 54controls the local oscillation frequency of the upconverting part tochange so that the frequencies of the pilot signals undergo sequentialvariations in the operating band of the power amplifier 37 while at thesame time the controller 54 correspondingly changes the oscillationfrequency of the local oscillator 45 in the frequency downconvertingpart 40 to convert the distortion components of the pilot signals to thebase band. In this embodiment the upconverting part 33 is configured toperform upconversion in two stages. That is, a local oscillator 33A1 ofa variable frequency f_(IF), a mixer 33B1 and a band-pass filter 33C1perform the first-stage upconversion to convert the output from the DAconvert 31 to an IF signal. A local oscillator 33A2 of a fixed frequencyf_(c)′, a mixer 33B2 and a band-pass filter 33C2 perform thesecond-stage upconversion to convert the IF signal to a high-frequencysignal.

Since the local oscillation frequency f_(IF) for conversion to the IFsignal is sufficiently lower than the local oscillation frequency(carrier frequency ) f_(c) for upconversion in the case of FIG. 30, suchtwo-stage upconversion provides increased accuracy in setting thefrequency for upconverting the baseband signal that is the output fromthe DA converter 31. Theoretically, however, the upconverting part 33may be of a single-stage configuration as in the FIG. 30 embodiment.

Incidentally, in this embodiment a series connection of the phaseadjuster 23A the gain adjuster 24A in FIG. 30 is referred to as a vectoradjuster 234A in the digital predistorter 20, and also vector adjusters234B and 234C are each representative of a similar series connection;the same goes for other embodiment.

In this embodiment, too, the frequency characteristic compensators 28A,28B and 28C may be disposed at the input sides or both of the input andoutput sides of the third-, fifth- and seventh-order distortiongenerators 22A, 22B and 22C as indicated by the broken lines.

FIG. 33 is a flowchart showing a control procedure for determining thecharacteristics of the frequency characteristic compensators 28A, 28Band 28C in the FIG. 32 embodiment. This determination of thecharacteristics takes place during a non-signal transmission period.

Step S1: The frequency characteristic control part 53 sets the frequencyf_(IF) for converting the pilot signals to an IF signal in the localoscillator 33A1 of the frequency upconverting part 33.

Step S2: The pilot signals PL₁ and PL₂ are input to the digitalpredistorter 20. The pilot signals are then provided from the digitalpredistorter 20 to the DA converter 31 for conversion to analog form,after which they are subjected to the two-stage frequency upconversionin the upconverting part 33, thereafter being input as an RF signal tothe power amplifier 37.

Step S3: The output RF signal from the power amplifier 37 is dividedinto two, one of which is provided to the frequency downconverting part40 to generate pilot signal components containing distortion componentsin the baseband are generated.

Step S4: The distortion extractors 52A, 52B and 52C extract respectiveodd-order distortion components. At this time, the distortion componentsof each odd order are detected at frequencies upper and lower than thefundamental wave.

Step S5: The odd-order distortion controllers 52A, 52B and 52C controlthe phases and gains of the odd-order distortions by the vectoradjusters 234A, 234B, and 234C of the digital predistorter 20 in mannerto minimize the odd-order distortion components being detected. The setvalues in the frequency characteristic compensators 28A, 28B and 28C tominimize the odd-order distortion components are stored in the storagepart 55 in correspondence to the respective odd-order distortioncomponents. The odd-order distortion components may be adjusted to besmaller than a certain set value. And the set values may be set byexternal setting means like a keyboard.

Step S6: The frequency characteristic control part 53 makes a check todetermine whether the repeat count of a series of processing in steps S1through S5 has reached a predetermined value, that is, whether thefrequency sweep has been completed, and if so, ends the setting of thefrequency characteristics.

Step S7: If it is determined in step S6 that the frequency sweep is notcomplete, increment the set frequency f_(IF) to f_(IF)+Δf, and return tostep S1 to repeat the series of steps S1 through S5.

Thus stored values are interpolated to obtain a frequencycharacteristic, which is set to the frequency characteristic compensatorin the same manner as described with the tenth embodiment.

In the FIG. 32 embodiment, too, the frequency characteristiccompensators 28A, 28B and 28C may be disposed at the input sides or bothof the input and output sides of the respective odd-order distortiongenerators 22A, 22B and 22C as indicated by the broken lines. When thefrequency characteristic compensators 28A, 28B and 28C are disposed atboth input and output sides of the odd-order distortion generator 22A,22B and 22C, the processing of FIG. 33 is performed separately for theinput- and output-side frequency characteristic compensators to settheir characteristics.

As referred to previously in respect of FIGS. 16A and 16B, thenonlinearity of the power amplifier is determined by the relationship ofnonlinearity dependence between the input and output sides. When thegate side (input side) and the drain side (output side) exert differentinfluence on the frequency characteristics of the intermodulationdistortions by the power amplifier, the provision of the frequencycharacteristic compensator only at the input or output side of eachodd-order distortion generator may sometimes make it difficult to renderthe frequency characteristics of the frequency characteristiccompensator obtained by the FIG. 3 procedure sufficiently inverse to thefrequency characteristics of the intermodulation distortions by thepower amplifier. By disposing the frequency characteristic compensatorat either side of each odd-order distortion generator and controlling itindependently of the others, it is possible to obtain characteristicsfor flattening the frequency characteristics at either side of the poweramplifier, that is, frequency characteristics inverse to those of theintermodulation distortions by the power amplifier.

FIG. 34 illustrates in block form a modified form of the FIG. 32embodiment. In this embodiment the pilot signals and the transmissionsignal are predistorted by independent digital predistorters 20 ₁ and 20₂ as in the FIG. 10 embodiment. The digital predistorters 20 ₁ and 20 ₂are identical in construction with the predistorter 20 in the FIG. 3embodiment. The output from the digital predistorter 20 ₂ for the pilotsignals is converted by a DA converter 31 ₂ to an analog signal, whichis frequency converted in a first frequency converting part 34 to afrequency band different from that of the transmission signal. By sweepcontrol of the set frequency of the local oscillator 34A of thefrequency upconverting part 34 by the digital predistorter control part50, the pilot signal frequency in the working band of the poweramplifier 37 is sweep-controlled. The two digital predistorters 20 ₁ and20 ₂ simultaneously control parameters in the digital predistortercontrol part 50.

In this way, the digital predistorter can be configured taking intoaccount the frequency characteristics of the power amplifier.

EFFECT OF THE INVENTION

As described above, according to the present invention, the pilot signalcomponents are extracted directly from the output of the power amplifier37 and the odd-order distortion components of a power series model ofthe digital predistorter are directly feedback-controlled—this permitsimplementation of a linear power amplifier with small secular andtemperature variations.

Moreover, since the odd-order distortions generated by the odd-orderdistortion generators are compensated for by frequency characteristicsinverse to those of the power amplifier, distortions by the poweramplifier can be canceled over a wide band.

The present invention produces such effects as listed below.

(1) High-precision distortion compensation can be achieved.

(2) Simple configuration is possible.

(3) A miniature transmitter can be offered.

(4) The distortion compensation can be held optimal for temperature orsecular variations.

1. A linear power amplifier comprising: a digital predistorter suppliedwith a digital transmission signal, for predistorting said digitaltransmission signal by use of a power series model to generate apredistorted signal; a DA converter for converting said predistortedsignal from said digital predistorter into an analog predistortedsignal; a frequency upconverting part for upconverting said analogpredistorted signal to a transmit frequency band; a power amplifier forpower-amplifying said upconverted signal; a frequency downconvertingpart for downconverting a portion of the output from said poweramplifier to output a downconverted signal; and a digital predistortercontrol part for extracting distortion components of the same odd ordersas those of said power series model and for controlling coefficients ofsaid digital predistorter in a manner to lower the levels of saidodd-order distortion components, wherein said digital predistorterincludes distortion generating paths each containing a series connectionof a distortion generator for generating one of distortions based onsaid power series model and a frequency characteristic compensator, andan adder for adding odd-order distortions from said distortiongenerating paths to said digital transmission signal and for outputtinga combined output as said predistorted signal, said digital predistortercontrol part includes means for controlling frequency characteristics ofsaid frequency characteristic compensators based on said extractedodd-order distortion components, and said frequency characteristiccompensators are formed by FIR filters whose frequency characteristicsare controlled by said extracted odd-order components.
 2. The linearpower amplifier of claim 1, which further comprises a pilot signalgenerator for generating a digital pilot signal for input to saiddigital predistorter, and wherein said digital predistorter control partincludes means for extracting odd-order distortion components of saiddigital pilot signal and for controlling the coefficients of saiddigital predistorter based on said extracted odd-order distortioncomponents.
 3. A linear power amplifier comprising: a first digitalpredistorter supplied with a digital transmission signal, forpredistorting said digital transmission signal by use of a power seriesmodel to generate a predistorted signal; a first DA converter forconverting said predistorted signal from said first digital predistorterinto an analog predistorted signal; a first frequency upconverting partfor upconverting said analog predistorted signal to a transmit frequencyband; a power amplifier for power-amplifying said upconverted signal;and a frequency downconverting part for downconverting a portion of theoutput from said power amplifier to output a downconverted signal, apilot signal generator for generating a digital pilot signal; a seconddigital predistorter supplied with said digital pilot signal, forpredistorting said digital pilot signal by use of a power series modelto generate a predistorted pilot signal; a second DA converter forconverting said predistorted pilot signal to an analog predistortedpilot signal; a second frequency upconverting part for upconverting saidanalog predistorted pilot signal by use of a predetermined frequency; acombiner for combining the output from said second frequencyupconverting part and said analog predistorted signal, and for inputtingsaid combined signal to said first frequency upconverting part; and adigital predistorter control part for extracting distortion componentsof the same odd orders as those of said power series model and forcontrolling coefficients of said digital predistorter in a manner tolower the levels of said odd-order distortion components.
 4. A linearpower amplifier comprising: a first digital predistorter supplied with adigital transmission signal, for predistorting said digital transmissionsignal by use of a power series model to generate a predistorted signal;a first DA converter for converting said predistorted signal from saidfirst digital predistorter into an analog predistorted signal; a firstfrequency upconverting part for upconverting said analog predistortedsignal to a transmit frequency band; a power amplifier forpower-amplifying said upconverted signal; a frequency downconvertingpart for downconverting a portion of the output from said poweramplifier to output a downconverted signal; a pilot signal generator forgenerating a digital pilot signal; a second digital predistortersupplied with said digital pilot signal, for predistorting said digitalpilot signal by use of a power series model to generate a predistortedpilot signal; a second DA converter for converting said predistortedpilot signal to an analog predistorted pilot signal; a second frequencyupconverting part for upconverting said analog predistorted pilot signalto a send frequency band by use of a predetermined second frequencydifferent from said first frequency; a combiner for combining the outputfrom said first frequency upconverting part and the output from saidsecond frequency upconverting part, and for inputting said combinedoutput to said power amplifier; and a digital predistorter control partfor extracting distortion components of the same odd orders as those ofsaid power series model and for controlling coefficients of said digitalpredistorter in a manner to lower the levels of said odd-orderdistortion components.
 5. The linear power amplifier of any one ofclaims 2, 3 and 4, wherein said first frequency upconverting partconverts said digital pilot signal to a frequency different from thefrequency of said transmission signal.
 6. The linear power amplifier ofany one of claims 2, 3 and 4, wherein said digital predistorterincludes: delay means for delaying said digital pilot signal and saiddigital transmission signal; distortion generating means for generating,in said digital pilot signal and said digital transmission signal, oneor more predetermined odd-order ones of the distortion componentsexpressed by a power series model; and adding means for combining saidodd-order distortion components and the output from said delay means toprovide said predistorted signal.
 7. The linear power amplifier of anyone of claims 2, 3 and 4, wherein said frequency downconverting partincludes an AD converter for converting said digital pilot signalcomponent to a digital signal.
 8. The linear power amplifier of claim 5,wherein said digital pilot signal is a combined version of two tonesignals of different frequencies but of the same level.
 9. The linearpower amplifier of claim 5, wherein said digital pilot signal is amodulated signal of a band narrower than that of said transmissionsignal.
 10. The linear power amplifier of any one of claims 2, 3 and 4,wherein said digital predistorter control part includes: a distortioncomponent extracting part for detecting said predetermined one or moreodd-order ones of the distortion components, expressed by a power seriesmodel of said digital pilot signal, from said digital pilot signalcomponent; and odd-order distortion characteristic control part forcontrolling, based on said detected distortion components, phases andamplitudes of the corresponding one or more predetermined odd-orderdistortion components to be generated by said digital predistorter. 11.The linear power amplifier of any one of claims 2, 3 and 4, wherein saiddigital predistorter control part includes: delay means for generating adelayed digital pilot signal from said digital pilot signal; distortiongenerating means for generating distortions of other odd orders thansaid predetermined odd orders from said digital pilot signal;subtracting means for subtracting said delayed digital pilot signal andsaid distortions of said other odd orders from said digital pilot signalcomponent to detect said desired odd-order distortion components; and anodd-order distortion characteristic control part for controlling, basedon said detected odd-order distortion components, phases and amplitudesof the corresponding one or more predetermined odd-order distortioncomponents to be generated by said digital predistorter.
 12. A linearpower amplifier comprising: a digital predistorter supplied with adigital transmission signal, for predistorting said digital transmissionsignal by use of a power series model to generate a predistorted signal;a DA converter for converting said predistorted signal from said digitalpredistorter into an analog predistorted signal; a frequencyupconverting part for upconverting said analog predistorted signal to atransmit frequency band; a power amplifier for power-amplifying saidupconverted signal; a frequency downconverting part for downconverting aportion of the output from said power amplifier to output adownconverted signal; a digital predistorter control part for extractingdistortion components of the same odd orders as those of said powerseries model and for controlling coefficients of said digitalpredistorter in a manner to lower the levels of said odd-orderdistortion components; a pilot signal generator for generating a digitalpilot signal for input to said digital predistorter, and wherein saiddigital predistorter control part includes means for extractingodd-order distortion components of said digital pilot signal and forcontrolling the coefficients of said digital predistorter based on saidextracted odd-order distortion components; a band separator forseparating a predistorted transmission signal component and apredistorted pilot signal from said predistorted signal, and forinputting said predistorted transmission signal component to said DAconverter; a second DA converter for said predistorted pilot signalcomponent to an analog predistorted pilot signal component; a secondfrequency upconverting part for upconverting said analog predistortedpilot signal component to said send frequency band by use of a secondfrequency different from a frequency used by said frequency upconvertingpart; and an adder for combining the output from said DA converter andthe output from said second frequency upconverting part, and forinputting said combined output as said predistorted signal to saidfrequency upconverting part.
 13. A linear power amplifier comprising: adigital predistorter supplied with a digital transmission signal, forpredistorting said digital transmission signal by use of a power seriesmodel to generate a predistorted signal; a DA converter for convertingsaid predistorted signal from said digital predistorter into an analogpredistorted signal; a frequency upconverting part for upconverting saidanalog predistorted signal to a transmit frequency band; a poweramplifier for power-amplifying said upconverted signal; a frequencydownconverting part for downconverting a portion of the output from saidpower amplifier to output a downconverted signal; a digital predistortercontrol part for extracting distortion components of the same odd ordersas those of said power series model and for controlling coefficients ofsaid digital predistorter in a manner to lower the levels of saidodd-order distortion components; a pilot signal generator for generatinga digital pilot signal for input to said digital predistorter, andwherein said digital predistorter control part includes means forextracting odd-order distortion components of said digital pilot signaland for controlling the coefficients of said digital predistorter basedon said extracted odd-order distortion components; a band separator forseparating a predistorted transmission signal component and apredistorted pilot signal from said predistorted signal, and forinputting said predistorted transmission signal component to said DAconverter; a second DA converter for said predistorted pilot signalcomponent to an analog predistorted pilot signal component; a secondfrequency upconverting part for upconverting said analog predistortedpilot signal component to said send frequency band by use of a secondfrequency different from a frequency used by said frequency upconvertingpart; and an adder for combining the output from said DA converter andthe output from said second frequency upconverting part, and forinputting said combined output as said predistorted signal to said poweramplifier, wherein said frequency downconverting part downconverts saidextracted pilot signal by use of said second frequency.
 14. A linearpower amplifier comprising: a digital predistorter supplied with adigital transmission signal, for predistorting said digital transmissionsignal by use of a power series model to generate a predistorted signal;a DA converter for converting said predistorted signal from said digitalpredistorter into an analog predistorted signal; a frequencyupconverting part for upconverting said analog predistorted signal to atransmit frequency band; a power amplifier for power-amplifying saidupconverted signal; a frequency downconverting part for downconverting aportion of the output from said power amplifier to output adownconverted signal; and a digital predistorter control part forextracting distortion components of the same odd orders as those of saidpower series model and for controlling coefficients of said digitalpredistorter in a manner to lower the levels of said odd-orderdistortion components, wherein said digital predistorter includesdistortion generating paths each containing a series connection of adistortion generator for generating one of distortions based on saidpower series model and a frequency characteristic compensator, and anadder for adding odd-order distortions from said distortion generatingpaths to said digital transmission signal and for outputting saidcombined output as said predistorted signal; an said digitalpredistorter control part includes means for controlling frequencycharacteristics of said frequency characteristic compensators based onsaid extracted odd-order distortion components, and said frequencycharacteristic compensators each include: a Fourier transformer fortransforming a time domain digital signal to a frequency domain digitalsignal; a coefficient multiplier for multiplying said frequency domaindigital signal by a coefficient based on one of said odd-orderdistortion components; and an inverse Fourier transformer fortransforming the output from said coefficient multiplier to a timedomain digital signal.
 15. The linear power amplifier of claim 1 or 14,wherein said digital predistorter includes: a linear transfer path andsaid distortion generating path to which said digital transmissionsignal is divided; a gain adjuster and a phase adjuster disposed at theoutput side of said distortion generator on said distortion generatingpath, for adjusting amplitudes and phases of said odd-order distortions;a delay device disposed in said liner transfer path; and a combiner forcombining the output from said linear transfer path and the output fromsaid distortion generating path, and for outputting the combined outputas said predistorted signal; and wherein said digital predistortercontrol part includes an odd-order distortion characteristic controlpart for controlling said gain adjuster and said phase adjuster toadjust the amplitudes and phases of said odd-order distortions.
 16. Thelinear power amplifier of claim 1 or 14, further comprising a pilotsignal generator for generating a adigital pilot signal of a banddifferent from the band of said transmission signal, and for providingsaid digital pilot signal to said digital predistorter, wherein saiddigital predistorter control part extracts odd-order distortions of saiddigital pilot signal as said odd-order distortion components.
 17. Thelinear power amplifier of claim 1 or 14, further comprising: a pilotsignal generator for generating a adigital pilot signal; another digitalpredistorter having the same configuration as that of said digitalpredistorter and supplied with said digital pilot signal; another DAconverter for converting the output from said another digitalpredistorter to an analog signal; another frequency upconverting partfor upconverting the output from said another DA converter to a banddifferent from the band of said transmission signal; and a combiner forcombining the output from said DA converter and the output from saidanother frequency upconverting part, and for providing said combinedoutput to said frequency upconverting part; and wherein said digitalpredistorter control part extracts odd-order distortion components ofsaid digital pilot signal as said odd-order distortion components. 18.The linear power amplifier of claim 16, wherein said digital pilotsignal is a combined version of two tone signals of differentfrequencies but of the same level.
 19. The linear power amplifier ofclaim 16, wherein said digital pilot signal is a modulated signal of aband narrower than the band of said transmission signal.
 20. The linearpower amplifier of claims 1 or 14, further comprising a pilot signalgenerator for generating a digital pilot signal and a second digitalpilot signal each having the same amplitude, said two digital pilotsignals being input to said digital predistorter and thence to saidpower amplifier via said DA converter and said frequency upconvertingpart, and wherein said digital predistorter control part includes: adistortion component detecting part for detecting, as said odd-orderdistortion components, intermodulation distortion components resultingfrom amplification of said two digital pilot signals by said poweramplifier; and a frequency characteristic control part for estimatingfrequency characteristics of a transmission route from saidintermodulation distortion components detected by said distortioncomponent detecting part, and for controlling frequency characteristicsof said frequency characteristic compensators.
 21. The linear poweramplifier of claim 20, wherein said digital predistorter control partincludes a frequency controller for controlling said digital pilotsignal generator to change the frequency interval between said twodigital pilot signals.
 22. The linear power amplifier of claim 20,wherein said frequency upconverting part includes a local oscillator forgenerating a variable frequency local signal for upconverting saidanalog predistorted signal by a variable frequency, and said digitalpredistorter control part includes a frequency controller for causingsaid two digital pilot signals to perform discontinuous frequency sweepin the operating band of said power amplifier by discontinuous frequencysweep of the oscillation frequency of said local oscillator.
 23. Thelinear power amplifier of claim 14, wherein said frequencycharacteristic compensators are each disposed at the input and/or outputside of the corresponding distortion generator.
 24. A digitalpredistortion method using the linear amplifier recited in any one ofclaims 2, 3 and 4, said method comprising the steps of: (a) generatingsaid digital pilot signal; (b) combining said digital pilot signal andsaid digital transmission signal, generating distortion components of apredetermined number of odd-orders based on a power series model, andadding said odd-order distortion components to generate a predistortedsignal; (c) converting said predistorted signal to an analogpredistorted signal; (d) upconverting said analog predistorted signal tothe send frequency band by a predetermined carrier frequency; (e) poweramplifying said upconverted signal; (f) downconverting a portion of saidpower-amplified output signal and outputting a pilot signal component;and (g) controlling coefficients of said digital predistorter based onsaid digital pilot signal component so that levels of said odd-orderdistortion components by said power series model become lower.
 25. Themethod of claim 24, wherein said step (g) includes a step of repeatedlyadjusting the coefficients of said digital predistorter so that thelevel ratios of said odd-order distortion components to saidtransmission signal becomes smaller go down below a predetermined value.26. The method of claim 24, wherein said step (a) includes a step ofgenerating, as said digital pilot signal, two digital tone signals ofthe same level but of different frequencies.
 27. A predistortion methodusing the linear power amplifier of claim 20, said method comprising thesteps of: (a) setting the frequency interval between said two digitalpilot signals; (b) measuring upper- and lower-side distortion componentsof said two digital pilot signals from the output from said poweramplifier; (c) comparing said upper- and lower-side distortioncomponents with preset reference values, determining gains and phases ofthe corresponding frequencies of said frequency characteristiccompensators so that said upper- and lower-side distortion componentsbecome smaller than said reference values, and storing values of saiddetermined gains and phases in storage means; (d) repeating said steps(a), (b) and (c) a plurality of times while changing said frequencyinterval between said two digital pilot signals for each round of steps;(e) obtaining frequency characteristics of gains and phases byinterpolation from said values of the gains and phases for respectivefrequencies stored in said storage means; and (f) setting said frequencycharacteristics of said gain and phases in said frequency characteristiccompensators provided in said distortion generating paths.
 28. Apredistortion method using the linear power amplifier of claim 20, saidmethod comprising the steps of: (a) setting local oscillation frequencyof said frequency upconverting part; (b) measuring distortion componentsof said two digital pilot signals from the output from said poweramplifier; (c) comparing said measured distortion components with presetreference values, determining gains and phases of the correspondingfrequencies of frequency characteristic compensators so that saiddistortion components become smaller than said reference values, andstoring said values of the determined gains and phases in storage means;(d) repeating said steps (a), (b) and (c) a plurality of times whilechanging said frequency interval between said two digital pilot signalsfor each round of steps; (e) obtaining frequency characteristics ofgains and phases by interpolation from said values of the gains andphases for respective frequencies stored in said storage means; and (f)setting said frequency characteristics of said gain and phases in saidfrequency characteristic compensators.
 29. The predistortion method ofclaim 27, further comprising a step of setting phase adjusters and gainadjusters on said distortion generating path so that said measureddistortion components become smaller than predetermined fixed values.30. A linear power amplification method comprising the steps of: (a)inputting a digital signal and a digital pilot signal to a digitalpredistorter, and adding said digital signal and said digital pilotsignal with a predetermined number of odd-order distortion componentsbased on a power series model to generate a predistorted signal; (b)converting said predistorted signal to an analog predistorted signal;(c) upconverting said analog predistorted signal to a send frequencyband by use of a predetermined carrier frequency; (d) power amplifyingsaid upconverted signal; (e) downconverting a portion of saidpower-amplified output signal to extract odd-order distortioncomponents; and (f) controlling coefficients of said digitalpredistorter so that the level ratios of said odd-order distortioncomponents to a transmission signal each become smaller than apredetermined value, wherein said step (a) includes the steps of:generating said digital pilot signal; and combining said digital pilotsignal and a digital transmission signal, and outputting said combinedoutput as said digital signal, and said step (a) is the step ofcombining two digital tones signals of different frequencies but of thesame level to generate said digital pilot signal, and said step (e) is astep of extracting odd-order distortion components of said digital pilotsignal.
 31. A linear power amplification method comprising the steps of:(a) inputting a digital signal and a digital pilot signal to a digitalpredistorter, and adding said digital signal and said digital pilotsignal with a predetermined number of odd-order distortion componentsbased on a power series model to generate a predistorted signal; (b)converting said predistorted signal to an analog predistorted signal;(c) upconverting said analog predistorted signal to a send frequencyband by use of a predetermined carrier frequency; (d) power amplifyingsaid upconverted signal; (e) downconverting a portion of saidpower-amplified output signal to extract odd-order distortioncomponents; and (f) controlling coefficients of said digitalpredistorter so that the level ratios of said odd-order distortioncomponents to a transmission signal each become smaller than apredetermined value, wherein said step (a) includes the step ofcontrolling frequency characteristics of said odd-order distortioncomponents by frequency characteristic compensators, and said step (f)includes the step of repeatedly adjusting coefficients of said frequencycharacteristic compensators so that the level ratio of said extractedodd-order distortion components to said transmission signal level becomesmaller than predetermined values.
 32. A linear power amplificationmethod comprising the steps of: (a) inputting a digital signal and adigital pilot signal to a digital predistorter, and adding said digitalsignal and said digital pilot signal with a predetermined number ofodd-order distortion components based on a power series model togenerate a predistorted signal; (b) converting said predistorted signalto an analog predistorted signal; (c) upconverting said analogpredistorted signal to a send frequency band by use of a predeterminedcarrier frequency; (d) power amplifying said upconverted signal; (e)downconverting a portion of said power-amplified output signal toextract odd-order distortion components; and (f) controllingcoefficients of said digital predistorter so that the level ratios ofsaid odd-order distortion components to a transmission signal eachbecome smaller than a predetermined value, wherein said step (f) furtherincludes the step of repeatedly controlling gains and phases of saidodd-order distortion components by said digital predistorter in a mannerto decrease the levels of said extracted odd-order distortioncomponents.
 33. The linear power amplifier of claim 17, wherein saiddigital pilot signal is a combined version of two tone signals ofdifferent frequencies but of the same level.
 34. The linear poweramplifier of claim 17, wherein said digital pilot signal is a modulatedsignal of a band narrower than the band of said transmission signal. 35.The predistortion method of claim 28, further comprising a step ofsetting phase adjusters and gain adjusters on said distortion generatingpath so that said measured distortion components become smaller thanpredetermined fixed values.